fuzzgen: Add invalid input counter

[afonso360]

👋 Hey,

This is a followup to a suggestion that @jameysharp pointed out in #5820 (review). With the refactoring done there we can now count how many inputs we reject.

This is great since we can at least keep track of this and hopefully improve it in the future. Right now it isn't great at about 74% rejected inputs:

== FuzzGen Statistics  ====================
Valid Inputs: 130000
Invalid Inputs: 373838 (74.2% of Total Inputs)
Total Runs: 705910
Successful Runs: 690177 (97.8% of Total Runs)
Timed out Runs: 8247 (1.2% of Total Runs)
Traps:
        user code: int_divz: 6448 (0.9% of Total Runs)
        user code: int_ovf: 566 (0.1% of Total Runs)
        user code: bad_toint: 472 (0.1% of Total Runs)

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